This invention relates to devices and methods for coating substrates.
Electrostatic spray coating typically involves atomizing a liquid and depositing the atomized drops in an electrostatic field. The average drop diameter and drop size distribution can vary widely depending on the specific spray coating head. Other factors such as the electrical conductivity, surface tension and viscosity of the liquid also play an important part in determining the drop diameter and drop size distribution. Representative electrostatic spray coating heads and devices are shown in, e.g., U.S. Pat. Nos. 2,685,536; 2,695,002; 2,733,171; 2,809,128; 2,893,894; 3,486,483; 4,748,043; 4,749,125; 4,788,016; 4,830,872; 4,846,407; 4,854,506; 4,990,359; 5,049,404; 5,326,598; 5,702,527 and 5,954,907. Devices for electrostatically spraying can-forming lubricants onto a metal strip are shown in, e.g., U.S. Pat. Nos. 2,447,664; 2,710,589; 2,762,331; 2,994,618; 3,726,701; 4,073,966 and 4,170,193. Roll coating applicators are shown in, e.g., U.S. Pat. No. 4,569,864, European Published Patent Application No. 949380 A and German OLS DE 198 14689 A1.
In general, the liquid sent to the spray coating head breaks up into drops due to instability in the liquid flow, often at least partially influenced by the applied electrostatic field. Typically, the charged drops from electrostatic spray heads are directed by electric fields towards an article, endless web or other substrate that moves past the spray head. In some applications, the desired coating thickness is larger than the average drop diameter, the drops land on top of one other, and they coalesce to form the coating. In other applications, the desired coating thickness is smaller than the average drop diameter, the drops are spaced apart at impact, and the drops must spread to form a continuous voidless coating.
Devices for electrostatically spraying can-forming lubricants onto a metal strip are shown in, e.g., U.S. Pat. Nos. 3,726,701; 4,073,966 and 4,170,193. In U.S. Pat. No. 3,726,701, the electrostatic potential is adjusted based on the speed and deposition rate of the article to be coated.
U.S. Pat. No. 2,733,171 employs mechanical oscillation of an electrostatic spray head and intermittent movement of the spray head electrostatic discharge wire in order to reduce striping or ribbing of the deposited coating material.
U.S. Pat. No. 5,049,404 employs piezoelectric vibration of a dielectric electrostatic spray nozzle in order to stabilize the surface shape of the liquid leaving the nozzle, reduce nozzle clogging at low flow rates and obtain extremely thin coatings.
Our copending U.S. patent application Ser. No. 09/841,380, filed Apr. 24, 2001 entitled ELECTROSTATIC SPRAY COATING APPARATUS AND METHOD and incorporated herein by reference discloses an apparatus and methods for applying a liquid coating to a substrate by electrostatically spraying drops of the liquid onto a liquid-wetted conductive transfer surface, and transferring a portion of the thus-applied liquid from the transfer surface to the substrate to form the coating.
Our copending U.S. patent application Ser. No. 09/757,955 filed Jan. 10, 2001 entitled COATING DEVICE AND METHOD and incorporated herein by reference discloses devices and methods for improving the uniformity of a wet coating on a substrate. The coating is contacted at a first position with the wetted surfaces of two or more periodic pick-and-place devices, and re-contacted at positions on the substrate that are different from the first position and not periodically related to one another with respect to their distance from the first position. The coating can be applied using point source nozzles such as airless, electrostatic, spinning disk and pneumatic spray nozzles and line source atomization devices. The nozzle or nozzles can be oscillated back and forth across the substrate.
The apparatus, devices and methods of the above-mentioned applications can provide very uniform coatings, especially when used in combination.
The present invention also provides an improvement in coating uniformity. In one aspect, the invention provides a method for forming a liquid coating on a substrate, comprising:
a) spraying a pattern of drops of the liquid onto a substrate from an electrostatic spray head that produces the pattern in response to an electrostatic field; and
b) repeatedly electrically altering the electrostatic field during spraying, thereby repeatedly changing the pattern.
A preferred method comprises spraying the pattern of drops onto a conductive transfer surface, and transferring a portion of the thus-applied liquid from the transfer surface to the substrate to form the liquid coating.
In another aspect, the invention provides a method for forming a liquid coating on a substrate, comprising:
a) spraying a pattern of drops of the liquid onto the substrate or onto a transfer surface from an electrostatic spray head that produces the pattern in response to an electrostatic field;
b) repeatedly changing the pattern in a first direction; and
c) in either order:
i) when a transfer surface is employed, transferring a portion of the thus-applied coating from the transfer surface to the substrate; and
ii) contacting the coating with two or more pick-and-place devices that improve the uniformity of the coating in a second direction.
The invention also provides an apparatus comprising an electrostatic spray head that produces a pattern of drops and a wet coating on a substrate in response to an electrostatic field, and a device or circuit for repeatedly electrically altering the electrostatic field during spraying, thereby repeatedly changing the pattern. In a preferred embodiment, the device or circuit changes the pattern in a first direction and the apparatus further comprises two or more pick-and-place devices that can periodically contact and re-contact the wet coating to improve the uniformity of the coating in a second direction.
The methods and apparatus of the invention can provide substantially uniform thin film or thick film coatings, on conductive, semi-conductive, insulative, porous or non-porous substrates. The apparatus of the invention is simple to construct, set up and operate, and can easily be adjusted to alter coating thickness and coating uniformity.